This invention relates generally to cathode ray tubes, and more particularly to tension mask assemblies having detensioning features.
A color cathode ray tube, or CRT, includes an electron gun for forming and directing three electron beams to a screen of the tube. The screen is located on the inner surface of the faceplate panel of the tube and is made up of an array of elements of three different color-emitting phosphors. A shadow mask, which may be either a formed mask or a tension mask having strands, is located between the electron gun and the screen. The electron beams emitted from the electron gun pass through apertures in the shadow mask and strike the screen causing the phosphors to emit light so that an image is displayed on the viewing surface of the faceplate panel.
One type of CRT has a tension mask comprising a set of strands that are tensioned onto a mask support frame to reduce their propensity to vibrate at large amplitudes under external excitation. Such vibrations would cause gross electron beam misregister on the screen and would result in objectionable image anomalies to the viewer of the CRT.
The mask stress required to achieve acceptable vibration performance is below the yield point of the mask material at tube operating temperature. However, at elevated tube processing temperatures, the mask""s material properties change and the elastic limit of the mask material is significantly reduced. In such a condition, the mask stress exceeds the elastic limit of the mask material and the material is inelastically stretched. When the tube is cooled after processing, the strands are longer than before processing and the mask frame is incapable of tensing the mask strands to the same level of tension as before processing. Another common problem with tension mask frame assemblies occurs when the mask strand material has a lower coefficient of thermal expansion than the mask support frame material. In such a case, tension on the mask strand increases during thermal processing causing more inelastic strain.
One detensioning system utilizes a dual compliant mask frame having a pair of support blade members centrally mounted on opposite sides of a frame wherein, tension is relieved at the center of the support blade member. Detensioning of the tension mask strands at the center when using a dual compliant frame may result in relatively greater tension on the strands toward the outsides or edges of the blade. In order to achieve a more uniform detensioning, in addition to the detensioning at the center, further detensioning is required at the edges of the mask.
It is therefore desirable to develop a mask frame assembly that allows the pattern and degree of the mask detensioning to be adjusted during the thermal cycle used in the manufacturing process of a CRT.
A tension mask assembly is provided having a frame consisting of two long sides disposed parallel to a central major axis and two short sides disposed parallel to a central minor axis which is orthogonal to the major axis. Support blade members are fixed to the frame along a central location of the long sides. Each support blade member includes an inner edge having sides formed of relatively low coefficient of thermal expansion material and an outer edge having at least one side or detensioning member formed of a relatively high coefficient of thermal expansion material. The centroid of each edge is separated from each other along the central minor axis.